1. Field of the Invention
The present invention concerns a method for generation of an angiographic image using magnetic resonance technology in which the contrast of vessel structures is intensified by a contrast agent. The invention also concerns a magnetic resonance apparatus for implementing such a method.
2. Description of the Prior Art
Magnetic resonance technology has been increasingly used for generation of angiographic images since, relative to other medical imaging methods (such as, for example, radioscopy with x-rays or computed tomography) it exhibits, among other things, the advantage that patient and medical personnel are subject to no radiation exposure.
Magnetic resonance (MR) technology is a known technique with which images of the inside of an examination subject can be generated. The examination subject is positioned in a comparably strong, static, homogeneous basic magnetic field (field strength of 0.2 Tesla to 7 Tesla and more) in an MR apparatus so that the nuclear spins in the object become oriented along the basic magnetic field. To excite nuclear magnetic resonances, radio-frequency excitation pulses are radiated into the examination subject, the excited nuclear magnetic resonances are measured and MR images are reconstructed based on these nuclear magnetic resonances. For spatial coding of the measurement data, rapidly-switched gradient fields are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored as complex number values in a k-space matrix. An MR image can be reconstructed by a multi-dimensional Fourier transformation from the k-space matrix populated with such data values.
Since MR enables a soft tissue contrast that can be adjusted in many ways, it is also used in angiography since the imaged contrast can be selected such that vessel structures can be made differentiable from surrounding tissue. In order to increase the diagnostic significance of MR angiogram, a contrast agent (for example based on gadolinium) is often used. The contrast agent is injected into a vessel system of a patient so that it highlights the vessel system relative to surrounding tissue after subsequent propagation.
The propagation speed of the contrast agent depends on the vessel system to be examined and on the pathologies present therein. When the contrast agent diffuses, it is primarily located in arterial vessels during a first phase (known as the arterial phase) while venous vessels are not yet filled with the contrast agent. Only in a second phase (known as the equilibrium phase) has the contrast agent distributed enough so that it is located both in the arteries and in the veins of the vessel system. The arterial phase typically lasts some seconds until it is replaced by the equilibrium phase.
An angiography image in which both the arterial portion and the venous portion of the vessel system are imaged in a comparable manner is typically hard for a user to assess with regard to detecting pathologies, since the superimposition of arterial and venous structures often makes the pathologies to be detected unrecognizable. In the production of an angiogram it is therefore typically insured to that either purely arterial images or purely venous images are generated.
Given the generation of an angiogram by contrast agent-supported MR technology, a further problem occurs in the representation of the arterial phase. Since the MR technique requires relatively long image data acquisition times that can exceed the duration of the arterial phase of the contrast agent passage, it is often not possible to be able to complete the acquisition of the measurement data within the arterial phase, such that various methods exist that divide the measurement data to be acquired in different ways.
U.S. Pat. No. 6,556,856 discloses a method for generation of a time-resolved MR angiogram in which a time-resolved series of MR images with low resolution is acquired during the arterial phase and high-resolution MR images are acquired in the subsequent equilibrium phase. Low resolution and high resolution MR images are combined after subsequent segmentation of the low resolution temporal series of MR images and the high resolution MR images. The segmentation of the low resolution MR images ensues by a comparison of the temporal intensity curve of individual voxels of the low resolution series of MR images relative to their contrast ratio, using reference curves whose determination in turn requires a manual intervention of a user. In total the method requires both a manual intervention by a user and elaborate post-processing steps after an acquisition of the measurement data. The need therefore exists to further improve contrast agent-supported MR angiography methods.